In Grand County, Colorado, where I live, there happens to be a project to build just such an overpass happening right now:
https://www.codot.gov/projects/sh9wildlife
http://denver.cbslocal.com/2015/09/21/cdot-wildlife-overpass...
This stretch of highway is so populated with wildlife that we would often see a handful of new unfortunate strikes every morning, and most everyone around had a story of a near miss or a hit. Underpasses and overpasses are being utilized and a fence runs along both sides to funnel the animals to them.
A fence was put in place along I-70 near us and has been rather effective but there are no underpasses and no overpasses in that implementation. In our area, the wildlife need to travel to the Blue River on one side of the highway for water and back up the mountains on the other side of the highway to graze (and give birth, etc.)
Money was raised not only from government sources but from a local resident pledge drive as well as a very gracious benefactor who owns land in the affected area.